The pK(a) values of five Fischer carbene complexes of the general structure (CO)(5)Cr=C(OR)CHR'R " of two (CO)(4)(PR3)Cr=C(OCH3)CH3 type complexes and of one aminocarbene complex, (CO)(5)Cr=C(NCH- (CH3)CH2)CH3, were determined in acetonitrile by a spectrophotometric method. These pK(a) values span a range from 22.2 for (CO)(5)Cr=C(OCH3)CH3 to 27.6 for (CO)(5)Cr=C(NCH(CH3)CH2)CH3. Deprotonation of the carbene complexes was achieved with strong amine bases such as TMG, DBU, and BEMP. Attempts at measuring the pK(a) of some other carbene complexes failed either because of instability of the respective anions (e.g., (CO)(5)Cr=C(OCH3)CH(CH3)(2)) or because the pK(a) was too high for deprotonation to occur ((CO)(5)Cr= C(N(CH3)(2))CH3), Similarities and differences in the structural dependence of these acidities in acetonitrile compared to other solvents such as 50% acetonitrile-50% water, DMSO, and THF are discussed.